I-1: Igneous Process and Tectonics
Activity in 1994 and Plan in 1995
We made GPS measurements and gravity survey in West Jawa crossing the Lembang and Cimandiri faults.
We installed a seismic network at Gunter Volcano and a tiltmeter at Merapi Volcano. We collected volcanic gas samples at several volcanoes in Jawa Island.
The Lembang fault is topographically and geologically remarkable and runs about 10km north of Bandung in the EW direction. The Cimandiri fault is another geological fault and located to the west of the Lembang fault, reaching the popular resort coast, Pelabuhanratu.
We established 15 GPS stations in west Jawa and started GPS static measurements with the purpose of monitoring crustal movements including the movements of Lembang and Cimandiri faults and investigating their relation with volcanic activity in this region. We have carried out GPS measurements at Bojonglopang, Sagaranten, Pelabuhanratu, Ciawi, Kiaradua(LIPI), Cisaat and Lembang in August and September and at Tugu, Leles, Ciawi, Ciater, Cibodas and Nagreg in October and November 1994 with reference to GPS stations at the National Coordination for Survey and Mapping(BAKOSURTANAL) and Institute Teknologi Bandung(ITB).
The results show that the GPS stations north of Lembang and Cimandiri faults are displaced to the east with rates of about 20mm/year and the stations south of the faults move to the north with rates of 10-20mm/year. This tendency of movements agrees with expected crustal movements caused by the subduction of Indo-Australia plate under the Eurasian plate. Since the propagation error of microwave is large due to the active ionosphere and high water vapor pressure in the troposphere in this region, GPS measurements are necessary to be repeated hereafter.
In 1994 gravity measurements have been carried out at the GPS stations and other points to investigate vertical movements as well as crustal structure in this region.
International connection of gravity was also completed between Japan and Indonesia.
Study of the Lembang fault is a very significant subject from the point of disaster reduction because the densely populated city Bandung has developed on the plain lake deposits.
Gunter Volcano is an andesitic stratovolcano located 35km southeast of Bandung, west Jawa. We added 3 new seismic stations to the one seismic station, which had already been installed by Volcanological Survey of Indonesia (VSI). Thus the new network consisting of 4 seismic stations covers the volcano. This is the first time to monitor the hypocenter of volcanic earthquakes in Indonesia, except Merapi which is one of the most active volcanoes in the world. A newspaper in Indonesia reported the new seismic observation with its purpose and significance. Analyzing the data, volcanic earthquakes were located 2.5km beneath the summit crater and at the deeper part (540km) beneath the western flank. The distribution of the volcanic earthquakes coincides with the location of craters and calderas.
Large pyroclastic flows occurred at Merapi on 22 November 1994 as reported in the Newsletter No.1. Dr.Sukhyar, Chief of the Volcano Analysis Division of VSI, informed us that low frequency earthquakes had begun to occur at Merapi. So we moved a tiltmeter from the flank to a point near the summit on 1 November, 3 weeks before the pyroclastic flows. Since the signals from the tiltmeter was not telemetered to the Merapi Volcano Observatory (MVO), we could not obtain the data. Unfortunately, the rainy season had already begun and it was difficult to access the summit of Merapi of almost 3,000 meters elevation. Mr. Subandryio, a researcher of ground deformation in MVO, succeeded to get data in the last February. The tilt records show inflation of the lava dome a week before the pyroclastic flows and gradual subsidence of the dome after the flows.
In 1995 we will install a tiltmeter at Gunter and transmit the data to Gunter Volcano Observatory in order to investigate the relation between volcanic seismicity and ground deformations.
A workshop "Merapi Decade Volcano International Workshop" will be held on 5-9 October in 1995 in Jogyakarta by VSI associated with IDNDR. The 75th Anniversary of VSI Will be also celebrated by the workshop. A wide range of subjects from the activity of the volcano to hazard mitigation will be discussed. We will present the results obtained from the cooperative study between DPRl and VSI. We will make ground deformation surveys at Merapi as well.
